The present invention relates to curable epoxy/aromatic diamine resin systems containing a defined group of aromatic trihydroxy compounds as cure accelerators.
The three dimensional epoxy-amine network produced by the curing of amine/epoxy resin formulations is well known to exhibit desirable mechanical and thermal properties. As a result, amine curable epoxy resin systems have been widely used as coatings, adhesives, sealants and matrices for fiber-reinforced composites. For each application, the epoxy/amine resin formulation must possess a particular degree of reactivity. In many cases, the reaction rate must be increased, and cure accelerators are added. Typically, additives which increase the cure rate seriously degrade the mechanical and thermal properties of the cured resin. Thus, there is a need to increase the cure speed of the epoxy/amine formulations while maintaining or preferably increasing the high mechanical properties (such as tensile strength and modulus) which are obtained with the unaccelerated resin system. This improvement is particularly desirable in high performance applications such as composites.
Considerable efforts have accordingly been made to improve epoxy/amine resin systems by adding various additives thereto. C. A. May and Y. Tanaka, Epoxy Resins Chemistry and Technology, Marcel Dekker, New York, 1973, for example, describes the addition of various Lewis acids, Lewis bases, and numerous salts and complexes as accelerators for epoxy/amine systems. Such accelerators, while improving the cure speed, have been found to adversely effect mechanical properties due to homopolymerization of the epoxy groups, which is facilitated in the presence of such accelerators.
It has also been disclosed that various mono- and di-hydroxy substituted aromatic compounds are effective in increasing the cure speed of certain epoxy resins. For example, Schechter et al in Industrial and Engineering Chemistry, Volume 48, No. 1, pages 94 to 97, 1956, disclosed that phenol was more effective than aliphatic alcohols in accelerating the reaction of phenyl glycidyl ether with diethylamine. Bowen et al in the American Chemical Society Advances in Chemistry Series, Volume 92, pages 48 to 59, 1970, disclosed that 4,4'-dihydroxydiphenyl sulfone, phenol, tetrabromobisphenol A and bisphenol A decreased the gel time of bisphenol A epoxy/triethylenetetramine systems with similar degrees of effectiveness.
Resorcinol, phenol, and various halogenated and nitrated derivatives of these compounds have also been found by Gough et al (Journal of Oil and Color Chemists Association, Volume 43, pages 409 to 418, 1960), Nagy (Adhesives Age, pages 20 to 27, April, 1967) and Partensky (American Chemical Society Advances in Chemistry Series, Volume 92, pages 29 to 47, 1970) to accelerate the cure of glycidyl epoxy/amine mixtures. In addition, Markovitz in "Chemical Properties of Crosslinked Polymers", American Chemical Society Symposium 1976, S. S. Labana, Ed., pages 49 to 58, has described the use of resorcinol and metal salts as coaccelerators for curable compositions containing cycloaliphatic epoxides.
While resorcinol and phenol thus have been found to provide desirable improvements in cure speed to certain epoxy/amine resin systems, further improvements in the cure speed of such systems, particularly with respect to cycloaliphatic epoxide/amine resin formulations, would be desirable.
In many epoxy/amine formulations, cycloaliphatic epoxides are used as the epoxy component since they impart improved mechanical and thermal properties to the cured compositions. For example, unreinforced castings of bis(2,3-epoxycyclopentyl)ether cured with m-phenylenediamine have tensile strengths and tensile moduli which are among the highest of any thermosetting material. Similarly, as described by McLean et al in Report No. 14450 of the National Research Council of Canada, November, 1974, high mechanical properties can be achieved in unreinforced castings made by curing 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylate with methylene dianiline. However, resin systems containing bis(2,3-epoxycyclopentyl)ether or 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylate cure more slowly with aromatic amines than similar compositions containing bisphenol A epoxy resins. This characteristic limits their utility in composite fabrication processes such as filament winding and reaction injection molding. Thus, there is a need for improved cure accelerators for cycloaliphatic epoxide/amine resin systems.
From Japanese Kokai No. 82/192428, published November 26, 1982, adhesive compositions comprising 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylate, triethanolamine borate and pyrogallol, in a weight ratio of 100:1:3, are known. Triethanolamine borate promotes the homopolymerization of the epoxy groups present in this formulation. Upon curing, the compositions of this publication primarily possess a crosslinked epoxy homopolymer structure, which possesses inferior mechanical properties as compared with thermosetting systems characterized by an epoxy-amine network structure, i.e., the epoxy/aromatic diamine formulations. As a result, the compositions of this reference have limited applications, and are not useful in high performance applications such as in composites.
German Offenlegungsschrift DE No. 2924717, published Jan. 10, 1980, describes the use of approximately stoichiometric amounts of pyrogallol to harden 1,3-diglycidyl-5,5-dimethylhydantoin. This system does not contain an aromatic amine curing agent. Moreover, in this system the pyrogallol does not function as a cure accelerator, but crosslinks with the epoxy resin. Thus, pyrogallol functions in this system as a curing agent producing a cured composition having an ether network structure.
British Pat. No. 1,054,045 describes the use of pyrogallol to harden bisphenol A type epoxies. In the compositions of this reference, an approximately stoichiometric amount of pyrogallol, with a small amount of methylene dianiline (1.5 parts per 100 parts of resin) as a co-curing agent is employed to crosslink a bisphenol A type epoxy having an epoxide equivalent weight of 500. Due to the large amount of pyrogallol present in this system, the pyrogallol functions as the primary crosslinking agent, leading to a cured composition having primarily an ether network structure.
While the prior art has thus described various mono- and dihydroxy aromatic compounds as cure accelerators for epoxy/amine resin systems, and has described the use of pyrogallol as a crosslinking agent (hardener) for epoxy resins, the prior art is devoid of any description of the use of aromatic trihydroxy compounds as cure accelerators for epoxy/aromatic diamine resin systems.
As is well known by those skilled in the art, cure accelerators function in a truly catalytic manner, enhancing the rate of reaction between the epoxy resin and the amine curing agent, without themselves appreciably reacting with the epoxy resin, as shown, e.g., by solvent extraction studies. This result is to be contrasted with the use of these compounds as curing agents wherein the trihydroxy aromatic compound is chemically incorporated into the resin network structure. The use of the trihydroxy aromatic compounds as hardeners (i.e., curing agents or crosslinkers) requires stoichiometric amounts (i.e., from approximately 0.4 to 1.0 equivalents of hydroxy per equivalent of epoxy groups in the epoxy resin), whereas cure acceleration is based on the use of only small quantities of the accelerator of from about 0.01 to about 0.35 equivalents of hydroxy per equivalent of epoxy groups in the epoxy resin, together with the conjoint presence of an aromatic diamine hardener in an amount within the general range of from about 0.4 to about 2.0 equivalents of amine N--H per equivalent of 1,2-epoxy groups in the epoxy resin. Cure acceleration is thus a quite different function from hardening, involving different quantities of additive, different modes of action and the presence of a primary hardener in preferably a stoichiometric excess.